1. Field of the Invention
This invention relates to the utilization of doped alumina for static charge sensitive applications, and to methods for making and using the same, e.g., for varying the electrical conductivity of alumina by doping with an appropriate transition metal oxide and subsequently heating to high temperatures in a reducing environment. This treatment allows the electrical conductivity to be tailored and thus provides a cost effective means for producing components with controlled resistivity.
2. Description of Related Art
Ceramics made of alumina are often utilized in various applications because they exhibit a wide variety of desirable properties such as high stiffness, low thermal expansion, chemical inertness, high wear resistance, reasonable strength and low cost. Aluminas are, however, electrical insulators, which makes them poor candidates for applications requiring electrical charge conduction. Electrostatic chuck dielectrics containing alumina and oxides of transition metals, such as titanium, have been suggested (U.S. Pat. No. 5,104,834 Watanabe, et al.; U.S. Pat. No. 5,151,845 Watanabe, et al.; U.S. Pat. No. 5,384,681 Kitabayashi, et al.; U.S. Pat. No. 5,384,682 Watanabe, et al.; U.S. Pat. No. 5,463,526 Mundt; European Pat. No. 0,339,903 Watanabe, et al.). Additionally, controlled resistivity aluminas are described for use in high vacuum, high voltage insulation applications (U.S. D.O.E. Contract DE-AC04-76DP00656, H. C. Miller, 1980).
However, it is believed that the use of controlled resistivity aluminas for the dissipation of electrical charges in ESD sensitive applications has not been previously disclosed. ESD sensitive applications include semiconductor circuit manufacturing, flat panel display manufacturing and magnetic recording head manufacturing. The use of fully insulating materials in these applications is not effective, because fully insulating materials do not allow static charges to be removed. Conversely, highly conductive materials, such as metals, allow the static charges to drain too quickly, potentially damaging sensitive devices with excessive electrical power.
Another area where charge dissipative materials are useful is in materials handling, in particular the handling of bulk materials in particulate form, such as grain production, sugar production, coal production, flour production, and textile production. Handling devices in these applications are often made from electrically conductive materials, such as metals, which may not possess some of the more desirable properties of ceramics, such as wear resistance, hardness, stiffness, or thermal stability.
The use of charge dissipative materials that are not fully insulative is also of value in applications requiring extremely low particulate concentrations in the vicinity of the material, since insulative materials tend to suffer from build up of static charge on their surfaces, which can attract and retain fine particulates. The electrostatic attractive forces between a charged object and a particle can be orders of magnitude larger than gravitational forces or aerodynamic forces, making it extremely difficult to remove fine particulate contamination from a charged surface. It has been shown that a 1 micron particle can generate an 80 kpsi bonding pressure with a surface charged to 1000V (Yost et al., Microcontamination, 6/86).
Accordingly, there remains a need in the art for materials having controlled resistivity between that of fully insulating materials and that of good conductors, for use as materials in applications requiring reduced electrostatic discharge damage, minimized charge accumulation, and reduced attraction of particulate contamination.
In one aspect, the invention relates to an alumina material having a resistivity ranging from about 104 to about 1014 ohm-cm. In one embodiment of this aspect of the invention, the alumina material comprises alumina that has been doped with one or more transition metal oxides. Typically, such a transition metal oxide doped alumina is fired in a reducing atmosphere during its preparation, and may also be fired in air prior to firing in a reducing atmosphere. In a particular embodiment of this aspect of the invention, the alumina material comprises alumina doped with titania. In a more particular embodiment of this aspect, the titanium oxide doped alumina is fired in a hydrogen atmosphere.
In another aspect, the invention relates to a coating material of controlled resistivity comprising the transition metal oxide doped alumina material dispersed on a substrate material.
In another aspect, the invention relates to an electrostatic charge dissipative material, and to articles made therefrom, comprising the transition metal oxide doped alumina. This material may be in the form of a coating applied to a substrate. The invention also relates to electrostatic charge dissipative articles made from or coated with the transition metal oxide doped alumina material.
In another aspect, the invention relates to a dielectric material comprising the transition metal oxide doped alumina.
In yet another aspect, the invention relates to apparatus for materials handling, and in particular for the handling of particulate materials, that is made from or has surfaces coated with the transition metal oxide doped alumina material described above.
In yet another aspect, the invention relates to a method of preparing the electrostatic charge dissipative ceramic material by combining alumina, one or more transition metal oxides, and, optionally, a sintering aid and a binder to form a mixture, forming the mixture into a green body, and sintering the green body in a reducing atmosphere for a period of time sufficient to provide a desired surface resistivity.
In yet another aspect, the invention relates to a method of dissipating electrostatic charge from a device by forming at least one part of the device from or coating at least one surface of at least one part of the device with a ceramic material comprising alumina doped with at least one transition metal oxide; and electrically connecting the alumina doped with at least one transition metal oxide to a ground or other material of lower electrical potential.
The invention provides an alumina material that has good strength, wear resistance, hardness, stiffness, and thermal stability (e.g., low thermal expansion), that is chemically inert, and that also provides a cost effective means for dissipating electrical charges in static sensitive applications, minimizing the risk of electrostatic discharge, as well as the risk of attracting particulate contamination in ESD and/or particulate sensitive applications, including semiconductor circuit manufacturing, flat panel display (e.g., liquid crystal display) manufacturing, magnetic recording head manufacturing, etc. The use of transition metal oxide doped aluminas allows the resistivity of the material to be controlled, so that when the material is grounded, static charge build up on the material is drained in a slow, controlled manner.